Seasonal variations of river and tidal flow interactions in a tropical estuarine system

The tidal hydrodynamics of a tropical estuarine system with river outflow seasonal variations was studied. Water level observations from eleven sites within the system were analyzed over one year in a restricted and shallow tropical estuary. The analysis was performed using wavelets, power spectral density analysis and a linear analytical model. This methodology gives the principal tidal components, their amplitude, phase and the interactions between the tide and the seasonal variations of the river outflow. The analytical model included the river discharge as an extra force on the frictional parameter to help understand the seasonal variations in tidal propagation within an estuary. The system was divided into three zones based on its bathymetry, geometry and river influence. Each zone presented different seasonal responses. River influence showed a 30% decrease in the amplitude of tidal signal from the mouth to the head, and the hydrodynamics were driven by the balance between the pressure gradient and frictional forces. This high tide attenuation in the river influence zone showed seasonal variations, therefore this research proposes adding into the frictional forces the river discharge to the frictional parameter in the analytical solution. The results show that although river discharge has a marked sub-tidal periodicity, river discharge has a strong influence on tidal hydrodynamics and inhibits the propagation of the tide toward the head of highly frictional estuarine systems.